This invention relates in general to rotary input devices or control knobs for controlling an electrical device. Rotary control knobs are commonly used to control a variety of electronic devices ranging from portable handheld devices to control panels such as those mounted in a vehicle for controlling audio, video, heating and ventilation, and navigation systems. The control knobs are mounted for rotary movement and are manually rotated by a user to control or provide some input to the electrical device. The user simply rotates the knob to a desired rotational position to provide an input for controlling the device in some predetermined manner.
One common control knob is a rotary electromechanical switch or encoder. A rotary electromechanical switch is a device that has a rotating shaft connected to one terminal capable of physically making or breaking a connection to one or more other terminals oriented in an arcuate or circular path. Because of the mechanical nature of the rotary electromechanical switch, the switch may break or malfunction over time from use.
It is also known to use a rotary control knob employing sensors instead of mechanical contact switches or potentiometers. The sensors detect the rotational movement of a knob and generate signals which are sent to a control device, such as a microprocessor, for determining the rotational position of the knob. Examples of known sensors for rotary control knobs include Hall effect sensors, photoelectric sensors, radio frequency sensors, and capacitive sensors. U.S. Pat. No. 5,936,613 discloses an embodiment of a rotary knob that uses a capacitive sensor assembly. The capacitor assembly includes a pair of arcuate traces that are oriented in a circular orientation about the rotational axis of the knob. The traces have varying widths along their length. An oscillator circuit is electrically connected to the traces. A conductive plate is secured to the knob and positioned in a slightly spaced relationship from the circular traces. As the knob is turned, the conductive plate moves in an arcuate path passing over the length of the traces and in cooperation with the oscillator circuit provides a signal representative of the position of the knob.